Optical add/drop wavelength switch

ABSTRACT

An optical add/drop wavelength switch is controllably changed from a bridge state, in which output is identical to input, e.g. a Wavelength Division Multiplexed (WDM) input, and an add/drop state, In which a signal input to an add port is substituted for a particular wavelength subrange of the WDM input, other wavelengths of the WDM input being unchanged. In one embodiment, the wavelength subrange of the WDM signal is given a polarization different from other wavelengths of the WDM, such as by using a stacked waveplate or other optical filter or polarization discriminator. The differently-polarized wavelengths can the be spatially separated, e.g. by a birefringent element or a polarization beam splitter, preferably In a bit-controlled fashion, such as by using a liquid crystal or other polarization controller. Polarization controllers and discriminators can be used similarly to selectably align or combine the add signal with the portion of the WDM signal outside the subrange. The add/drop wavelength switch can be used, e.g. In an optical token ring network and/or to make-up an optical crossbar for exchanging any arbitrarily designatable channels, e.g. among a plurality of multi-channel optical fibers.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation application of pending priorapplication Ser. No. 09/036,202, now U.S. Pat. No. 6,166,838, filed Mar.6, 1998, incorporated herein by reference, which claims priority fromU.S. Provisional patent application 60/042,373, filed Mar. 24, 1997titled “Optical Add/Drop Wavelength Switch” incorporated herein byreference.

GOVERNMENT INTERESTS

The invention was made with Government support under Contract DARPA II:DAAH01-97-C-R308 awarded by U.S. Army Missile Command, AMSMI-AC-CRAY,Redstone Arsenal, AL 35898. The Government has certain rights in theinvention.

The present invention relates, in general, to optical communicationsystems, and, more particularly, to an add/drop wavelength switch forwavelength division multiplex (WDM) optical communications.

BACKGROUND INFORMATION

Optical wavelength division multiplexing has gradually become thestandard backbone network for fiber optic communication systems. WDMsystems employ signals consisting of a number of different opticalwavelengths, known as carrier signals or channels, to transmitinformation on optical fibers. Each carrier signal is modulated by oneor more information signals. As a result, a significant number ofinformation signals may be transmitted over a single optical fiber usingWDM technology.

Despite the substantially higher fiber bandwidth utilization provided byWDM technology, a number of serious problems must be overcome, such as,multiplexing, demultiplexing, and routing optical signals, if thesesystems are to become commercially viable. The addition of thewavelength domain increases the complexity for network managementbecause processing now involves both filtering and routing. Multiplexinginvolves the process of combining multiple channels (each defined by itsown frequency spectrum) into a single WDM signal. Demultiplexing is theopposite process in which a single WDM signal is decomposed intoindividual channels. The individual channels are spatially separated andcoupled to specific output ports. Routing differs from demultiplexing inthat a router spatially separates the input optical channels into outputports and permutes these channels according to control signals to adesired coupling between an input channel and an output port.

Currently, filters based on fiber Bragg gratings (FBG) are among themost popular add/drop wavelength filters used in WDM networks for theadd/drop operation. Another approach in the add/drop operation is theuse of an array-waveguide-grating filter. In both cases, the add/dropoperation is always on, which, it is believed, is not a very effectiveway to utilize the optical channel. Although another 2×2 optical switchcan be integrated with the FBG such that an add/drop operation can becontrolled by a switching gate, this is fundamentally cumbersome andineffective. Furthermore, optical switches available in the market aremostly mechanical optical switches that are not suitable in networkwavelength routing because of their short lifetime (i.e., a moving motorwears out in time) and high power consumption. Although other types ofoptical switches are available, such as thermal optical switches,crosstalk in such switches is generally too high to permit large scalesystems to be provided.

SUMMARY OF THE INVENTION

The present invention combines the characteristics of add/drop operationof a filter and the switching capability of an optical switch. Theadd/drop wavelength switch has at least two input ports for the incomingWDM signal and the add signal, and at least two output ports for the WDMpass-through signal and the drop signal. The wavelength switch isoperated in two modes, referred to as the bridge state and add/dropstate, respectively. In the bridge state, the incoming WDM signalcontinuously flows through the optical node without being disturbed.When controlled to do so by either the local optical node or the WDMnetwork, the wavelength switch changes to the add/drop state in which apre-defined optical channel is dropped from the WDM signal and the addsignal is substituted into the WDM signal. The add signal can be asingle channel or multiple channels. A unique feature of this add/dropwavelength switch is that the pass-through channels are not disturbed bythe transition during switching between states. This assures theuninterrupted flow of WDM signals through the network. Based on thisfeature, an optical token ring can be realized in which multipleadd/drop wavelength switches are cascaded. An array of these add/dropswitches can be used to implement a wavelength crossbar that enablesoptical channels to be arbitrarily exchanged between multiple WDMnetworks.

An optical add/drop wavelength switch is controllably changed from abridge state, in which output is identical to input, e.g. a WavelengthDivision Multiplexed (WDM) input, and an add/drop state, in which asignal input to an add port is substituted for a particular wavelengthsubrange of the WDM input, other wavelengths of the WDM input beingunchanged. In one embodiment, the wavelength subrange of the WDM signalis given a polarization different from other wavelengths of the WDM,such as by using a stacked waveplate or other optical filter orpolarization discriminator. The differently-polarized wavelengths canthe be spatially separated, e.g. by a birefringent element or apolarization beam splitter, preferably in a bit-controlled fashion, suchas by using a liquid crystal or other polarization controller.Polarization controllers and discriminators can be used similarly toselectably align or combine the add signal with the portion of the WDMsignal outside the subrange. The add/drop wavelength switch can be used,e.g. in an optical token ring network and/or to make-up an opticalcrossbar for exchanging any arbitrarily designatable channels, e.g.among a plurality of multi-channel optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction withthe accompanying drawings, in which:

FIGS. 1a and 1 b are block diagrams illustrating the functionality ofthe add/drop wavelength switch in accordance with an embodiment of thepresent invention in the bridge state and the add/drop state,respectively;

FIGS. 2A and 2B show two examples of the spectra of the add/dropwavelength switch according to an embodiment of the present invention.FIG. 2a shows asymmetric spectra. FIG. 2b shows the spectra for anadd/drop wavelength switch with evenly spaced inter-digitalcharacteristics;

FIGS. 3a and 3 b illustrate in schematic form a 2-D add/drop wavelengthswitch according to an embodiment of the present invention in the bridgestate (FIG. 3a) and add/drop state (FIG. 3b);

FIGS. 4a and 4 b illustrate a schematic form of a 3-D add/dropwavelength switch according to an embodiment of the present invention inthe bridge state (FIG. 4a) and add/drop state (FIG. 4b);

FIG. 5 shows an optical token ring based on cascaded add/drop wavelengthswitches according to an embodiment of the present invention;

FIG. 6 shows a wavelength crossbar using an array of add/drop wavelengthswitches according to an embodiment of the present invention.

FIGS. 7a and 7 b illustrates in schematic form, an add/drop wavelengthswitch according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1a and 1 b illustrate in block diagram form the generalfunctionality of the present invention. As used herein, the term“channel” refers to a particular range of frequencies or wavelengthsthat define a unique information signal. Each channel is ideally evenlyspaced from adjacent channels, although this is not necessary. Unevenspacing may result in some complexity in design, but, as will be seen,the present invention can be adapted to such a channel system in amanner that will be clear to those of skill in the art afterunderstanding the present disclosure. This flexibility is important inthat the channel placement is driven largely by the technicalcapabilities of transmitters (i.e., laser diodes) and detectors, soflexibility is of significant importance.

In the illustration, laterally spaced symbols such as arrows 106 a-106 hand distinguished symbols 108 each indicate a channel with the inputsignal 111 made up of a number of channels. The WDM signal 111 is fed asan input through port 101 using conventional optical signal couplingtechniques to the add/drop wavelength switch 999. In the bridge stateshown in FIG. 1a, the WDM signal 111 passes through the wavelengthswitch 999 uninterrupted and exits at port 103 to be redirected back tothe WDM network. The add port 102 and drop port 104 are connected toform a “bridger” in which no add/drop operation occurs. The add signal113 that is input through the add port 102 passes through the wavelengthswitch 999 and exits at the drop port 104 as the drop signal 114.

In contrast, when the add/drop wavelength switch 999 is switched to theadd/drop state, a pre-defined optical channel 106 f is extracted fromthe WDM signal 111 and exits as the drop signal 114 at the drop port104. The add signal 113, on the other hand, is combined with the WDMsignal exiting at port 103 to the WDM network as shown in FIG. 1b.

The output spectra of the add/drop wavelength switch are designed basedon the function required by the WDM network. In FIGS. 2a and 2 b, twoexperimental examples are given to show evenly spaced add/drop channels(FIG. 2b) and asymmetrical channels (FIG. 2a) that can be realized byusing the composite-waveplate (CWP) technique disclosed in theApplicants' U.S. patent applications Ser. Nos. 08/739,424 and 08/780,291(now U.S. Pat. No. 5,694,233), incorporated herein by reference.

FIGS. 3a and 3 b illustrate the add/drop wavelength switch 999 inschematic form in two control states. In accordance with a preferredembodiment, the add/drop switch 999 is under binary control from acontrol bit and has two states (i.e., “bridge” or “add/drop”). In FIGS.3a and 3 b, bold solid lines indicate the optical paths for the fillspectrum of channels in the WDM input signal 700. Solid thin linesindicate the optical paths of signals comprising a primary subset ofchannels from the WDM signals that are to pass through the add/dropswitch 999 undisturbed. Thin-intermittent dashed lines indicate theoptical paths for the drop channels that comprise a secondary subset ofchannels. Thin dotted lines indicate the optical path for the addsignal. Finally, thick dotted lines are the optical paths for theprimary subset of channels from the WDM network combined with the addsignal. It is important to understand that each of these subsets maycomprise more than one channel and may itself be a set of WDM signals.Each of the lines representing optical paths are further labeled witheither a short perpendicular line indicating horizontal polarization, ora large dot indicating vertical polarization, or both a perpendicularline and a large dot indicating mixed horizontal and verticalpolarizations in the optical signal at that point.

In FIG. 3a, the WDM signals 700 and the add signal 701 enter a firstbirefringent element 200 that spatially separates the horizontal andvertically polarized components of these signals 700, 701. The firstbirefringent element 200 is made of a material that allows thevertically polarized portion of the optical signal to pass throughwithout changing course because they are ordinary waves in thebirefringent element 200. In contrast, horizontally polarized waves areredirected at an angle because of the birefringent walk-off effect. Theangle of redirection is a well-known function of the particularmaterials chosen. Examples of materials suitable for construction of thebirefringent elements used in the preferred embodiments include calcite,rutile, lithium niobate, YVO4 based crystals, and the like. Apolarization beam splitter can also be used to perform a similarfunction for polarization separation.

The vertically polarized components 312 from the input WDM signal 700are coupled into a fixed polarization rotator 301_1 such that the stateof polarization (SOP) becomes horizontal. The add signal 701 is coupledto a switchable (e.g. in response to a control bit) polarization rotator301_2 under control of a control bit. Switchable polarization rotator301_2 consists of two sub-element rotators 301-2 a, 301-2 b that form acomplementary state, i.e. when one turns on the other turns off. Rotator301_2 serves to selectively rotate the polarization of the add signal bya predefined amount. In the preferred embodiment, rotator 301_2 rotatesthe polarization of signals by either 0° (i.e., no rotation) or 90°. InFIGS. 3a and 3 b, gray-shaded areas indicate polarization rotation andwhite (transparent) areas indicate no polarization rotation. Theswitchable polarization rotator 301_2 can be made of one or more typesof known elements including parallel aligned nematic liquid crystalrotators, twisted nematic liquid crystal rotators, ferroelectric liquidcrystal rotators, pi-cell liquid crystal rotators, magneto-optic basedFaraday rotators, acousto-optic and electro-optic polarization rotators.Commercially available rotators using liquid crystal based technologyare preferred, although other rotator technologies may be applied tomeet the needs of a particular application. The switching speeds ofthese elements range from a few milliseconds to nanoseconds, andtherefore can be applied to a wide variety of systems to meet the needsof a particular application. These and similar basic elements areconsidered equivalents and may be substituted and interchanged withoutdeparting from the spirit of the present invention.

FIG. 3a illustrates the bridge state in which the signals exiting thepolarization rotators 301_1 and 301_2 have horizontal polarization asindicated by the short vertical lines to the right of the polarizationrotators 301-1, 301-2. A first stacked waveplates element 400 is made ofa stacked plurality of birefringent, composite waveplates at selectedorientations that generate two eigen states. The first eigen statecarries a first sub-spectrum with the same polarization as the input,and the second eigen state carries a complementary sub-spectrum at theorthogonal polarization. The polarization of the incoming beam and thetwo output polarizations form a pair of spectral responses, where (H, H)and (V, V) carry the first part of the input spectrum and (H, V) and (V,H) carry the complementary (second) part of the input spectrum, where Vand H are vertical and horizontal polarizations, respectively. Furtherdetails of the design and the filtering mechanism of the stackedwaveplates element are disclosed in the Applicants' U.S. patentapplication Ser. Nos. 08/739,424 and 08/780,291 (now U.S. Pat. No.5,694,233), incorporated herein by reference.

In the case of this add/drop wavelength switch, the first eigen statecarries the primary sub-spectrum (i.e., the undisturbed WDM signals)with the same polarization as the input, and the second eigen statecarries a complementary sub-spectrum (i.e., the add/drop channels) atthe orthogonal polarization.

The input WDM signal 700 is decomposed into two components orthogonalpolarizations when passing through the first stacked waveplates element400. The primary spectrum 500 is coded in the horizontal polarization412 and the drop spectrum 501 is coded in the vertical polarization 414.The add signal 701, has a horizontal polarization before entering thestacked waveplates element 400. It is rotated by 90° as it passesthrough the first stacked waveplates element 400, because it has thesame spectrum as the drop channel. At the plane after the first stackedwaveplates element 400 as shown in FIG. 3a, the add/drop channels arevertically polarized 416, while the primary spectrum is horizontallypolarized 412.

Optical signals 500, 501 and 502 represent the primary, drop and addsignals that are coupled to the second birefringent element 201. Thesecond birefringent element 201 has a similar construction to the firstbirefringent element 200 and serves to spatially separate thehorizontally and vertically polarized components of the optical signals500, 501 and 502. The two orthogonal polarizations that carry theprimary spectrum 500 in horizontal polarization and the add/dropspectrum 502, 501 in vertical polarization are separated by the secondbirefringent element 201 because of the birefringent walk-off effect.

A second set of polarization rotators 302_1 and 302_2 follow the secondbirefringent element 201. The primary signal 500 passes through afixed-type (non-switching) rotator 302_1 that rotates the polarizationby 90°. The add/drop signals 502, 501 pass through a switchablepolarization rotator 302_2, that, in the bridge state (FIG. 3a) is alsoset (e.g. in response to the control bit) to rotate the polarization by90°. At the exit plane of the polarization rotators 302_1 and 302_2, theprimary spectrum has vertical polarization 318 and the add/drop spectrahave horizontal polarization 322, as indicated in FIG. 3a.

Following the second set of polarization rotators 302_1 and 302_2, thepreceding components are repeated, but arranged in opposite order. Asshown for the bridge state in FIG. 3a, a third birefringent element 202recombines the primary spectrum 500 and the drop signal 501 because ofthe walk-off effect. Thus, in the bridge state, no add/drop operationoccurs. The add signal 502 propagates upward in the third birefringentelement 202 and keeps its horizontal polarization 324.

The second stacked waveplates element 401 has the same structure andcomposition as to the first stacked waveplates element 400. With thehorizontally polarized (324, 326) beams 501, 502 input to the secondstacked waveplates element 401, the add/drop spectrum is furtherpurified and rotates its polarization by 90°, 334. On the other hand,the vertically polarized (328) beam 500 (carrying the primary WDMsignals) input to the second stacked waveplates element 401 maintainsits polarization 332 but is also purified when it exits the secondstacked waveplates element 401. The 9° polarization rotation 334 of thehorizontally polarized beams 501, 502 is due to the fact that theadd/drop spectrum is the complementary state of the second stackedwaveplates element 401. At the output of second stacked waveplateselement 401, all four beams have vertical polarization 332, 334. Theupper two beams carry the full WDM spectrum and the lower two beamscarry the add signal's spectrum.

To recombine the two sets of beams, a third set of polarization rotators303_1 and 303_2 and a fourth birefringent element 203 are used. Again,the third set of polarization rotators consists of a fixed-typepolarization rotator 303_1 and a switchable (e.g. in response to thecontrol bit) polarization rotator 303_2. At least the second (and, ifdesired both) of the polarization rotators 303_1 and 303_2 have twosub-elements 303-1 a, 303-1 b, 303-2 a, 303-2 b that intercept the twosets of beams. The complete WDM signals carried by the upper two beams(indicated by the heavy solid lines after the third birefringent element202 in FIG. 3a) pass through the fixed polarization rotator 303_1 suchthat one of the upper beams has its polarization is rotated by 90° 342.The two orthogonal polarizations are then recombined by the fourthbirefringent element 203 that exits to port 103.

The two lower beams carrying the add signal pass through the switchablepolarization rotator 303_2 so that the polarization of one of the lowerbeams is rotated 344 by 90°. They are then recombined by the fourthbirefringent element 203. In this design, the sub-elements 303-1 a,303-1 b, 303-2 a, 303-2 b of the third set of polarization rotators303_1 and 303_2 are set at complementary states to the correspondingsub-elements 301-1 a, 301-1 b, 301-2 a, 301-2 b in the first set ofpolarization rotators 301_1 and 301_2. This complementary design assureshigh contrast operation for the polarization rotators and furtherassures high isolation for spectral filtering. This completes the bridgestate of operation for the add/drop wavelength switch 999.

In the add/drop state, the optical paths are shown in FIG. 3b. The threeswitchable polarization rotators 301_2, 302_2 and 303_2 have switched(e.g. in response to a change in the control bit) to their complimentarystates, i.e. from on to off or off to on, depending on their originalstates. In this state of operation, the light paths for the primaryspectrum 500 remain unchanged (compared to the bridge state). Thisdesign assures that the WDM signals that are not affected by theadd/drop operation flow through the optical node without beinginterrupted. This can be seen from the optical paths for the primaryspectrum 500 through the fixed polarization rotators 301_1, 302_1 and303_1 shown in FIGS. 3a and 3 b. The primary spectrum 500 passesundisturbed through the entire add/drop wavelength switch 999 along anoptical path that remains unchanged between the bridge state (FIG. 3a)and the add/drop state (FIG. 3b).

In contrast, the paths of the add signal and the drop signal areinterchanged between the add/drop state and the bridge state, i.e., thedrop signal 501 now exits at port 104 and the add signal 502 is combinedwith WDM primary signal 500 that exits through port 103. In FIG. 3b, theadd signal 701 is again decomposed into two orthogonal polarizations.Because the first polarization rotator 301_2 is now set to have theoutput polarizations all vertical 352, they pass through the firststacked waveplates element 400 which rotates the polarization by 90° sothat both components of the add signal 701 become horizontally polarized354. These horizontally polarized beams propagate upward in the secondbirefringent element 201 due to its extraordinary wave characteristic.The add signal 502 meets the drop signal 501 at the exit plane 356 ofthe second birefringent element 201. These two signals containing theadd/drop spectra then pass through the second polarization rotator302_2, which is set for no polarization rotation. The add signal 502continues to propagate upward through the third birefringent element 202and meets the primary WDM signal 500 at the exit plane 358 of the thirdbirefringent element 202. The drop signal 501, however, propagatesstraight through the third birefringent element 202 because it is anordinary wave in this birefringent element 202. It is clear up to thispoint that the add signal and the drop signal have exchanged their pathsin comparison to the bridge state shown in FIG. 3a.

These four beams pass through the second stacked waveplates element 401.The primary signal 500 keeps its polarization 362, 364 and the add/dropsignals 502, 501 rotate their polarizations by 90° 366, 367, 368, 369.They pass through the fourth set of polarization rotators 303_1 and303_2 such that orthogonal polarizations result 372, 374, 376, 378.These two sets of beams are recombined by the fourth birefringentelement 203 and exit to ports 103 and 104, respectively. This completesthe add/drop operation of the add/drop wavelength switch 999.

In the two-dimensional wavelength switch depicted in FIGS. 3a and 3 b,all the optical paths are laid within the same plane. However, thedesign concept of the present invention is not limited by thisstructure. An example showing a three-dimensional design is illustratedin FIGS. 4a for the bridge state and 4 b for the add/drop stateoperation. Two changes have been made with this structure as compared tothe 2-D design shown in FIGS. 3a and 3 b. In this embodiment, the firstand fourth birefringent elements 200 and 203 are oriented orthogonallyto the second and third birefringent elements 201 and 202. The secondchange is the arrangement of the sub-elements (or pixels) of thepolarization rotators, 301, 302 and 303.

FIG. 7A and 7B depict another embodiment providing an optical add/dropwavelength switch. FIG. 7A depicts the configuration in the bridge stateand FIG. 7B depicts the configuration in the add/drop state. Theadd/drop state differs, in its configuration, from the bridge state inthat, in the bridge state, a switchable liquid crystal polarizationcontroller 7012 is set in a non-rotating state while in the add/dropstate, the polarization controller 7012 is set (e.g., in response to achange in a control bit) to cause both horizontal and verticalpolarization to be rotated by 90 degrees. The effect will be describedbelow. As can be seen by comparing FIGS. 7A and 7B, it is possible usingthe depicted switch to achieve selectable add/drop functions bycontrolling only a single element, i.e. all other elements, includingthe polarization rotators, can be provided as fixed (non-switching)components.

In FIG. 7A, an input port 7014 receives a WDM input signal which, in thedepicted embodiment, includes a first wavelength (or range ofwavelengths) or channel 7016, as well as other channels 7018 a, 7018 b.The add port 7022 receives an add signal containing a frequency orfrequencies 7024 corresponding to those of the WDM signal with which theadd signal 7024 is to be exchanged in the add/drop state. In a mannersimilar to that described above in connection with FIGS. 3 and 4,birefringent elements or other polarization separators 7026 a, 7026 bspatially separate the horizontal and vertically polarized components ofthe signals 7022, 7014. A first polarization rotation 7028 a (such as apolymer or other polarization rotator) rotates the horizontallypolarized portion of the add signal to a vertical rotation so that bothcomponent beams are in a vertical orientation for polarization 7032 a. Apolarization rotator 7028 b is positioned to provide a 90 degreepolarization rotation to the vertically polarized component of the WDMsignal so that both beams of the WDM signal are horizontally polarized7032 b.

In a manner similar to that discussed above in connection with FIGS. 3and 4, a filter 7034 is configured to generate two eigen states asdescribed above. In the embodiment of FIG. 7A, the polarization of theadd signal is rotated from vertical 7032 a to horizontal 7036 a, whilethe WDM signal has the corresponding wavelength 7016 rotated to verticalpolarization 7036 b while other wavelengths 7018 a, 7018 b remain in ahorizontal polarization 7036 c.

A combination polarization beam separator and prism (PBS/P) 7038includes right angle prisms 7038 a, 7038 b, 7038 c, 7038 d and PBSs 7042a, 7042 b. The polarization beam separators are configured to reflect(e.g. at 90 degrees) components having a first polarization (in theillustrated embodiment, reflecting vertically polarized components)while transmitting horizontally oriented components. Examples ofpolarization beam separators that can be used In accordance with thepresent invention include those available from Nitto Optical Co. Ltd. ofJapan and those available from Lambda Research. Thus, in theconfiguration of FIG. 7a, the horizontally polarized add signal isreflected by right angle prism 7038 a, is transmitted through PBS 7042 ato right angle prism 7038 c, thence to right angle prism 7038 d, throughthe second PBS 7042 b and reflected by right angle prism 7038 b, e.g.back into its original path 7044. The horizontally polarized portion ofthe WDM signal 7036 c is transmitted straight through the first andsecond PBSs 7042 a, 7042 b along its original path 7046. The verticallypolarized component of the WDM signals 7036 b is reflected downward bythe first PBS 7042 a and thence reflected by the second and third prisms7038 c, 7038 d to the second PBS 7042 b where it is reflected back intoits original path 7046. Accordingly, when the switch is in its bridgestate as depicted in FIG. 7a, the signal output by the PBS/P 7044, 7946is identical to that which is input to the PBS/P 7038. Accordingly, thebridge function can now be achieved by using a second filter 7052, thirdand fourth polarization rotators 7054 a, brand third and fourthpolarization separator/combiners. 7056 a, b. Second filter 7052 provides90 degree rotation to components within a first wavelength range 7024,7016 while leaving other components 7018 a, 7018 b unrotated so theresult is a vertically oriented polarized add signal 7058 andhorizontally polarized WDM signal 7062. Polarization rotators 7054 a,7054 b are positioned to rotate one of the beams of the add signal to ahorizontal polarization 7064 a and rotate one of the beams of the WDMsignal to a vertical polarization 7064 b so that the polarizationseparator/combiner 7056 a, 7056 b will combine the signals to provide asingle drop signal 7066 at the first output (drop) portion 7068 a (whichwill be substantially identical to the input/add signal 7024) and asingle bridge output signal 7072 at the second output port 7068 b (whichis substantially identical to the input WDM signal 7016, 7018 a, 7018b).

As noted above, the configuration of FIG. 7B is substantially identicalto that of FIG. 7A except for the state of polarization controller 7012.Accordingly, the horizontally polarized portion of the WDM signal willbe transmitted straight through the first and second PBS components 7042a, 7042 b. Similarly, the signals reflected from the third prism 7038 dwill be identical to that described above in connection with FIG. 7A,i.e., the vertically polarized portion of the WDM signal and the addsignal components (which are horizontally polarized) will be reflectedupward by the third prism 7038 d to the polarization controller 7012.However, in FIG. 7B, the polarization controller 7012 is in an activestate such that it will rotate the vertically polarized portion of theWDM signal to a horizontal rotation and will also rotate thehorizontally polarized add signal to a vertical polarization.Accordingly, the add signal which, in the bridge configuration passedthrough the second PBS 7042 b, will, in the add/drop configuration ofFIG. 7B, be reflected by the second PBS 7042 b, in line with thehorizontally polarized portion of the WDM 7046′. The selected wavelengthrange from the WDM signal which in the bridge configuration of FIG. 7Awas reflected by the second PBS 7042 b, will, in the add/dropconfiguration of FIG. 7B be transmitted straight through the second PBSto the fourth prism 7038 d where it will be reflected into the path7044′ which, in the bridge configuration (FIG. 7a) was taken by the addsignal 7024. Thus, the selected range 7016 of the WDM signal and the addsignal 7024 have been caused to exchange paths, compared to their pathsin the (post PBS/P) bridge configuration. Accordingly, although theconfiguration and operation of the second filter 7052 polarizationrotator 7054 a, 7054 b and separator/combiners 7056 a, 7056 b areidentical to the configuration and operation in the bridge state (FIG.7A), the signal provided on the output port WDM 7068 b includes the addsignal 7024 while the output of the drop port 7068 a is the component7016 of the input WDM signal which formerly occupied the channel nowoccupied by the add signal 7024.

Several system applications can benefit from this invention. Forexample, FIG. 5, illustrates an optical token ring for circulatingoptical channels in a WDM network. In FIG. 5, laterally spaced barsindicate different optical channels. Each add/drop node 5012 a-d canextract or (drop) a pre-defined optical channel and substitute another(add) optical channel that is spectrally complementary to thepass-through channels. At least two features distinguish this add/dropwavelength switch from the prior art. First, a single wavelength switchcan extract multiple contiguous channels from the WDM network withoutcascading stages of filters, as are required with fiber Bragg gratingbased filters. Second, the pass-through WDM signals will not be affectedby the add/drop wavelength switch during the switching process. To ourknowledge, no such a device is available in the industry. This “nointerruption” design assures continuous information flow.

A “wavelength crossbar” based on this add/drop wavelength switch isillustrated In FIG. 6. Multiple add/drop wavelength switches 6012 a-6012p are cascaded to perform arbitrary wavelength channel exchange betweenmultiple WDM networks. In this example, four fibers 6014 a-d enter thewavelength crossbar. Each fiber carries four WDM wavelength channels fora total of 16 input channels. By controlling that state of each add/dropswitch 6012 a-6012 p (e.g. by providing a control bit to each switch),each of the four wavelength buses (designated “n bus” In FIG. 6) canexchange one of the optical channels between two of the fibers. In thecase of arbitrary wavelength exchange, three wavelength buses are neededfor each optical channel. Thus to permit arbitrary wavelength exchangeamong the 16 input channels, a total of 8 wavelength busses are used,each bus having 4 add/drop switches, for a total of 32 switches. Itshould be understood that each wavelength bus can handle a singlewavelength channel or a contiguous group of channels. This feature isbelieved to be not available with other technologies.

In light of the above description a number of advantages of the presentinvention can be seen. The present invention achieves optical add/dropswitching using only optical and solid state electronic components, andparticularly without the need for moving or mechanical components. Thepresent device provides for long average lifetime and low powerconsumption. The present device provides low cross-talk such as lessthan about −25 dB. The present invention can be provided without theneed for Filter Bragg Gratings or array-waveguide-grating filters. Thepresent invention can achieve operation in which the add/drop operationis not necessarily always on. The present invention is easilycontrollable and switchable between a bridge configuration and anadd/drop configuration, preferably by controlling solid state electronicdevices such as polymer polarization rotators and/or liquid crystalpolarization controllers and preferably in response to simple controlsignals such as using a single bit for controlling switching between abridge configuration and an add/drop configuration. The presentinvention can provide optical add/drop wavelength switches which arephysically small in lateral and three-dimensional size. The presentinvention can enable and facilitate effective and efficient opticaltoken ring systems (e.g. for circulating optical channels in a WDMnetwork). The present invention can provide wavelength crossbarfunctionality for exchanging any optical channels from multi-channelinputs (such as fiber optic inputs) e.g. between fibers in any desiredfashion so that any two input channels (or more) can be exchanged witheach other for providing to output ports or fibers.

A number of variations and modifications of the invention can be used.In general, the depicted vertical and horizontal polarizations can beswitched with one another. The switches can be configured to accommodatemore or fewer channels or groups of channels than those depicted.Although the embodiments depict configurations which are generallylinear from input ports to output ports, the components can be arranged(with appropriate redirection components such as prisms, mirrors and thelike) to define L-shaped, U-shaped or other light paths. Because of thegenerally symmetric nature of components in at least someconfigurations, it may be useful to redirect some or all signal paths topass through components more than once so as to reuse some or allcomponents. It may be useful, in some embodiments, to employ componentsin an order different from that depicted. In general, it is possible touse some features of the invention without using others such asproviding add/drop functionality without the final step of combiningspatially-separated vertically and horizontally polarized components(e.g. when the output is to be provided to another switch which can makeuse of already-separated vertically and horizontally polarizedcomponents, e.g. in a crossbar multi-switch configuration).

What is claimed is:
 1. An optical network, comprising: a plurality ofinterconnected add/drop nodes for processing at least one input opticalsignal comprising a plurality of wavelength channels and at least oneadd signal comprising at least one wavelength channel, wherein: at leastone of the add/drop nodes is configured in an add/drop state to (a)extract from the plurality of wavelength channels in the input opticalsignal a first subset of wavelength channels and form a pass-throughoptical signal comprising a second subset of wavelength channels that iscomplementary to the first subset of wavelength channels and (b) add tothe pass-through optical signal an add signal comprising an add set ofwavelength channels that is complementary to the second subset ofoptical channels, wherein during the add/drop state the pass-throughoptical signal flows continuously through the add/drop node; and atleast one of the add/drop nodes is configured in a bridge state tooutput the input optical signal and the add signal out of differentoutput ports and wherein, when an add/drop node changes from theadd/drop state to the bridge state or from the bridge state to theadd/drop state, at least a portion of the plurality of wavelengthchannels flows continuously through-the add/drop node.
 2. The opticalnetwork of claim 1, wherein each of the add/drop nodes includes a singleadd/drop switch.
 3. The optical network of claim 1, wherein each of theadd/drop nodes is free of cascading.
 4. The optical network of claim 1,wherein the first subset of wavelength channels includes multiplecontiguous channels and the flow of at least a portion of the pluralityof wavelength channels through a node is not interrupted when the nodeis in the add/drop state.
 5. The optical network of claim 1, wherein atleast a particular one of the add/drop nodes comprises: a first inputport that receives the at least one input optical signal; a second inputport that receives the add signal; at least a first output port; and aswitch being switchable between the add/drop state and a bridge state inresponse to a control signal, wherein, in the add/drop state, the switchextracts the first subset of wavelength channels from the at least oneinput optical signal and propagates the second subset of wavelengthchannels as the pass-through signal, combines the pass-through signalwith the add signal to form a combined optical signal output through thefirst output port and, in the bridge state, the switch outputs the firstsubset of wavelength channels and the second subset of wavelengthchannels of the at least one input optical signal through the firstoutput port.
 6. The optical network of claim 5, wherein the at least aparticular one of the add/drop nodes comprises at least a second outputport that is spatially displaced from the first output port, wherein, inthe add/drop state, the second output port outputs a drop signalincluding the first subset of wavelength channels in the at least oneinput optical signal and, in the bridge state, the second output portoutputs the add signal.
 7. The optical network of claim 6, wherein theswitch comprises: at least a first optical filter that receives at leasta portion of the at least one input optical signal and provides, atleast substantially simultaneously, an optical output in which the firstsubset of wavelength channels has a polarization different from thesecond subset of wavelength channels; at least a first displacementmember that receives the optical output and provides the pass-throughand drop signals, said drop signal comprising the first subset ofwavelength channels and wherein the pass-through and drop signals arespatially displaced with respect to one another; at least a firstpolarization rotator switchable between the bridge state and theadd/drop state, wherein, in the bridge state, the polarizations of thepass-through signal and the drop signal are either both rotated or bothunrotated and, in the add/drop state, the polarization of one of thepass-through and drop signals is not rotated; and at least a firstcombining member which displaces the add signal into at leastsubstantial alignment with the pass-through signal when the firstpolarization rotator is in the add/drop state and wherein the add signalis not in at least substantial alignment with the pass-through signalwhen the first polarization rotator is in the bridge state.
 8. A methodfor optical add/drop switching, comprising: receiving, at a first inputport, at least a first optical input comprising at least first andsecond wavelength range components, corresponding, respectively, tofirst and second wavelength ranges; receiving, at a second input port,an optical add signal comprising at least the first wavelength range;receiving at least one add/drop control signal; in response to theadd/drop control signal, extracting said first wavelength rangecomponent from the first optical input to form a pass-through signalcomprising the second wavelength range component; receiving, at thefirst input port, at least a second optical input including at leastthird and fourth wavelength range components corresponding,respectively, to third and fourth wavelength ranges; receiving, at thesecond input port, an optical add signal including at least the thirdwavelength range; receiving at least one bridge control signal; and inresponse to the bridge control signal, outputting said second opticalinput and said optical add signal through spatially displaced outputs.9. The method of claim 8, wherein the pass-through signal is combinedwith the add signal.
 10. The method of claim 8, wherein, in said bridgeand add/drop states the first wavelength range component and add signalhave different polarizations.
 11. The method of claim 9, wherein, in theadd/drop state, the add signal is displaced into at least substantialalignment with respect to the pass-through signal and the firstwavelength range component is displaced out of at least substantialalignment with respect to the pass-through signal.
 12. An opticaladd/drop wavelength switch, comprising: a first input port that receivesat least a first optical input including at least first and secondwavelength range components corresponding, respectively, to first andsecond wavelength ranges; a second input port that receives an opticaladd signal including at least the first wavelength range; at least afirst output port; a switch fabric being switchable between an add/dropmode and a bridge mode in response to a control signal, wherein, in theadd/drop mode, the switch fabric extracts said first wavelength rangecomponent from the first optical input and propagates the secondwavelength range component as a pass-through signal, combines thepass-through signal with the add signal to form a combined opticaloutput, and outputs the combined optical output through the first outputport and, in the bridge mode, the switch fabric outputs the first andsecond wavelength range components of said first optical input throughsaid first output port; and at least a second output port that isspatially displaced from the first output port, wherein, in the add/dropmode, the second output port outputs the first wavelength rangecomponent and, in the bridge mode, the second output port outputs theadd signal.
 13. The optical add/drop wavelength switch of claim 12,wherein the switch fabric comprises: at least a first optical filterthat receives at least a portion of the at least a first optical inputand provides, at least substantially simultaneously, an optical outputin which the first wavelength range component has a polarizationdifferent from the second wavelength range component; at least a firstdisplacement member that receives the optical output and provides thepass-through and drop signals, said pass-through signal comprising thesecond wavelength range component and said drop signal comprising thefirst wavelength range component and wherein the pass-through and dropsignals are spatially displaced with respect to one another; at least afirst polarization rotator switchable between the bridge mode and theadd/drop mode, wherein, in said bridge mode, the polarization of saidpass-through signal and drop signal are either both rotated or bothunrotated and, in said add/drop mode, the polarization of one of saidpass-through and drop signals is not rotated; and at least a firstcombining member which displaces the add signal into at leastsubstantial alignment with said pass-through signal when said firstpolarization rotator is in the add/drop mode and wherein the add signalis not in at least substantial alignment with said pass-through signalwhen said first polarization rotator is in the bridge mode.
 14. Theoptical add/drop wavelength switch of claim 13, wherein the firstoptical filter includes a plurality of stacked birefringent elements.15. The optical add/drop wavelength switch of claim 13, wherein thefirst displacement member includes a polarization beam separator. 16.The optical add/drop wavelength switch of claim 12, wherein the switchfabric comprises: at least a first polarization separator for receivingthe first optical input and spatially separating the first optical inputinto an optical signal component with a first polarization and anoptical component with a second, different polarization.
 17. The opticaladd/drop wavelength switch of claim 16, wherein the switch fabriccomprises: an optical signal polarization rotator for changing thepolarization of one of said first and second optical signal components.18. The optical add/drop wavelength switch of claim 17, wherein theswitch fabric comprises: at least a first add signal polarizationseparator for receiving the add signal and spatially separating the addsignal into a first add signal component with a first polarization and asecond add signal component with a second, different polarization. 19.The optical add/drop wavelength switch of claim 18, wherein the switchfabric comprises: an add signal polarization rotator for changing thepolarization of one of said first and second add signal components. 20.The optical add/drop wavelength switch of claim 19, wherein at least oneof said optical signal polarization rotator and said add signalpolarization rotator is controllable between a first state, changing ahorizontal polarization to a vertical polarization, and a second state,changing a vertical polarization to a horizontal polarization.
 21. Theoptical add/drop wavelength switch of claim 20, wherein at least one ofsaid optical signal polarization rotator and said add signalpolarization rotator is controlled to change state whenever said firstpolarization rotator is controlled to change state.
 22. An opticaladd/drop switch, comprising: a first input port for receiving at least afirst optical input including at least first and second wavelength rangecomponents corresponding to first and second wavelength ranges,respectively; a second input port for receiving an optical add signalincluding at least the second wavelength range; means for receiving atleast one add/drop control signal; means for extracting said firstwavelength range component from the first optical input in response tothe at least one add/drop control signal, such that the secondwavelength range component forms a pass-through signal in response tothe add/drop control signal; means for combining the pass-through signalwith the add signal in response to the add/drop control signal; andwherein the first input port receives at least a second optical inputincluding at least third and fourth wavelength range componentscorresponding to third and fourth wavelength ranges, respectively, andthe second input port receives a second optical add signal including atleast the third wavelength range and further comprising, in response toa bridge control signal, means for outputting said second optical inputand said second optical add signal through separate outputs.
 23. Anoptical add/drop wavelength device, comprising: at least a first inputport for receiving at least a first optical input, the first opticalinput including first and second wavelength ranges; at least a secondinput port for receiving at least a first optical add signal, the firstoptical add signal including at least the first wavelength range; anoptical switch in communication with the first and second input ports,the switch being configured in an add/drop mode to extract the firstwavelength range from the first optical signal to form a pass-throughoptical signal comprising the second wavelength range, and to add theadd signal to the pass-through optical signal to form a combined opticaloutput, wherein in the add/drop mode the pass-through optical signalpasses continuously through the switch; at least a first output portwherein, in the add/drop mode, the first output port outputs thecombined optical output and, in a bridge mode, the first optical outputoutputs the first optical input; and at least a second output port thatis spatially displaced from the first output port, wherein, in theadd/drop mode, the second output port outputs the first wavelength rangecomponent of the first optical input and, in the bridge mode, the secondoutput port outputs the add signal.
 24. The optical add/drop wavelengthdevice of claim 23, wherein the switch comprises: at least a firstoptical filter that receives at least a portion of the at least a firstoptical input and provides, at least substantially simultaneously, anoptical output in which the first wavelength range of the at least afirst optical input has a polarization different from the secondwavelength range of the at least at a first optical input; at least afirst displacement member that receives the optical output and providesthe pass-through signal and a drop signal comprising the firstwavelength range and wherein the pass-through and drop signals arespatially displaced with respect to one another; at least a firstpolarization rotator switchable between the bridge mode and the add/dropmode, wherein, in said bridge mode, the polarization of saidpass-through signal and drop signal are either both rotated or bothunrotated and, in said add/drop mode, the polarization of one of saidpass-through and drop signals is not rotated; and at least a firstcombining member which displaces the add signal into at leastsubstantial alignment with said pass-through signal when said firstpolarization rotator is in the add/drop mode and wherein the add signalis not in at least substantial alignment with said pass-through signalwhen said first polarization rotator is in the bridge mode.
 25. Theoptical add/drop wavelength device of claim 24, wherein the firstoptical filter includes a plurality of stacked birefringent elements.26. The optical add/drop wavelength device of claim 24, wherein thefirst displacement member includes a polarization beam separator. 27.The optical add/drop wavelength device of claim 24, wherein the switchcomprises: at least a first polarization separator for receiving thefirst optical input and spatially separating the first optical inputinto an optical signal component with a first polarization and anoptical signal component with a second, different polarization.
 28. Theoptical add/drop wavelength device of claim 27, wherein the switchcomprises: an optical signal polarization rotator for changing thepolarization of one of said first and second optical signal components.29. The optical add/drop wavelength device of claim 28, wherein theswitch comprises: at least a first add signal polarization separator forreceiving the add signal and spatially separating the add signal into afirst add signal component with a first polarization and a second addsignal component with a second, different polarization.
 30. The opticaladd/drop wavelength device of claim 29, wherein the switch comprises: anadd signal polarization rotator for changing polarization of one of saidfirst and second add signal components.
 31. The optical add/dropwavelength device of claim 30, wherein at least one of said opticalsignal polarization rotator and said add signal polarization rotator iscontrollable between a first state, changing a horizontal polarizationto a vertical polarization, and a second state, changing a verticalpolarization to a horizontal polarization.
 32. The optical add/dropwavelength device of claim 31, wherein at least one of said opticalsignal polarization rotator and said add signal polarization rotator iscontrolled to change state whenever said first polarization rotator iscontrolled to change state.
 33. A method for optical add/drop switching,comprising: receiving, at a first input port, at least a first opticalinput, the first optical input comprising first and second wavelengthranges; receiving, at a second input port, at least a first optical addsignal, the optical add signal including at least the first wavelengthrange; extracting the first wavelength range from the first opticalsignal to form a pass-through optical signal comprising the secondwavelength range; combining the pass-through optical signal with the addsignal, wherein in the extracting and combining steps, the secondwavelength range of the first optical input signal flows continuouslyalong an optical path; receiving, at the first input port, at least asecond optical input including at least third and fourth wavelengthrange components; receiving, at the second input port, an optical addsignal including at least the third wavelength range; receiving at leastone bridge control signal; and in response to the bridge control signal,outputting said second optical input and said optical add signal throughspatially displaced outputs.
 34. The method of claim, 33, wherein thepass-through signal is combined with the add signal.
 35. The method ofclaim 33, wherein, in an add/drop state, the add signal is displacedinto alignment with the pass-through signal and the first wavelengthrange component of the first optical signal is displaced out ofalignment with the pass-through signal.
 36. An optical add/drop switch,comprising: at least a first input port which receives at least a firstoptical input; at least a second input port which receives an opticaladd signal; at least a first optical filter which receives at least aportion of said first optical signal and provides a differentiatedsignal in which at least a first wavelength range has a polarizationdifferent from other wavelengths of said first optical signal; at leasta first displacement member which receives said differentiated signaland provides a member first output and a member second output, saidmember first output including said first wavelength range, said membersecond output including said other wavelengths and wherein said memberfirst and second outputs are spatially displaced with respect to oneanother; at least a first polarization rotator switchable between anadd/drop state and a bridge state, wherein, in said bridge state, thepolarization of said member first and second outputs are either bothrotated or both unrotated and, in said add/drop state, the polarizationof one of said member first and second outputs is rotated and thepolarization of the other of said first and second outputs is notrotated; and at least a first combining member which displaces said addsignal into at least substantial alignment with said member secondoutput when said first polarization rotator is in said add/drop stateand wherein said add signal is not in at least substantial alignmentwith said member second output when said first polarization rotator isin said bridge state.
 37. The optical add/drop switch of claim 36,wherein the first optical filter includes a plurality of stackedbirefringent elements.
 38. The optical add/drop switch of claim 36,wherein the first displacement member includes a polarization beamseparator.
 39. The optical add/drop switch of claim 36, wherein theswitch comprises: at least a first polarization separator for receivingthe first optical input and spatially separating the first optical inputinto an optical signal component with a first polarization and anoptical signal component with a second, different polarization.
 40. Theoptical add/drop switch of claim 39, wherein the switch comprises: anoptical signal polarization rotator for changing the polarization of oneof said first and second optical signal components.
 41. The opticaladd/drop switch of claim 40, wherein the switch comprises: at least afirst add signal polarization separator for receiving the add signal andspatially separating the add signal into a first add signal componentwith a first polarization and a second add signal component with asecond, different polarization.
 42. The optical add/drop switch of claim41, wherein the switch comprises: a polarization rotator for changingthe polarization of one of said first and second add signal components.43. The optical add/drop switch of claim 42, wherein at least one ofsaid optical signal polarization rotator and said add signalpolarization rotator is controllable between a first state, changing ahorizontal polarization to a vertical polarization, and a second state,changing a vertical polarization to a horizontal polarization.
 44. Theoptical add/drop switch of claim 43, wherein at least one of saidoptical signal polarization rotator and said add signal polarizationrotator is controlled to change state whenever said first polarizationrotator is controlled to change state.
 45. An optical add/drop switch,comprising: at least a first input port which receives at least a firstoptical input; at least a second input port which receives an opticaladd signal; at least a first polarization separator for receiving atleast said first optical input and spatially separating said firstoptical input into a first optical signal component with a firstpolarization and a second optical signal component with a second,different polarization; at least a first optical filter which receivesat least a portion of each of said first and second optical signalcomponents and provides corresponding differentiated optical signals inwhich at least a first wavelength range has a polarization differentfrom other wavelengths of said first and second optical signalcomponents; at least a second polarization separator for receiving atleast said add signal and spatially separating said add signal into afirst add signal component with a first polarization and a second addsignal component with a second, different polarization; at least a firstdisplacement member which receives at least one of said differentiatedoptical signals and at least one of said first and second add signalcomponents and provides at least member first, second, and thirdoutputs, said member second and third outputs including said firstwavelength range and said member first output including said otherwavelengths, and wherein said member first, second, and third outputsare spatially displaced with respect to one another; at least a firstpolarization rotator switchable between an add/drop state and a bridgestate, wherein, in said bridge state, the polarization of said memberfirst, second, and third outputs are either both rotated or bothunrotated and, in said add/drop state, the polarization of one of saidmember first and second outputs is rotated and the polarization of theother of said first and second outputs and said member third output isnot rotated; and at least a first combining member which displaces saidat least one of said first and second add signal components into atleast substantial alignment with said member first output when saidfirst polarization rotator is in said add/drop state and wherein said atleast one of said first and second add signal components is not in atleast substantial alignment with said member first output when saidfirst polarization rotator is in said bridge state.